This invention relates to a thin-film semiconductor element and particularly to the construction of a thin-film semiconductor element to be used as in an active matrix type liquid crystal display device.
In recent years, thin-film transistors (hereinafter referred to briefly as "TFT") have been copiously used as components as for active matrix type liquid crystal display devices and image sensors. As a natural consequence, the desirability of developing a TFT capable of manifesting improved semiconducting characteristics when used in liquid crystal display devices has been finding recognition.
The construction of the conventional TFT represented by a bottom-gate staggered type TFT will be described below with reference to FIG. 2. FIG. 2A is a plan view, FIG. 2B a cross section taken through FIG. 2A along a line II--II, and FIG. 2C a cross section taken through FIG. 2A along a line IV--IV.
An insulating substrate 7 made of glass is overlaid by a gate electrode layer 3 and further overlaid sequentially by a gate insulation layer 4, a semiconductor layer 5, and a channel protecting layer 1 in the order mentioned. After the channel protecting layer 1 has been formed so as to obtain a predetermined channel length, a contact layer 6, a source electrode layer 2, and a drain electrode layer 2 are formed. The source electrode layer and the drain electrode layer per se are short-circuited by the contact layer 6. The contact layer 6 on the channel protecting layer 1, therefore, is removed as masked by the source electrode layer 2 and the drain electrode layer 2. Here, the source electrode layer 2 and the drain electrode layer 2 are formed with a width narrower than the width of the channel protection layer 1. This statement equals the expression W.sub.0 &gt;W.sub.1 wherein W.sub.1 stands for the width of the source electrode layer or drain electrode layer and W.sub.0 for the width of the channel protection layer. The reason for this relation is that the resistance between the source electrode layer and the drain electrode layer must be lowered by giving a greater width to the channel layer. Further, for the purpose of ensuring the protection of the channel layer, it is necessary that the alignment of the component layers should be facilitated by allowing the channel protection layer to have a greater width.
These problematic points of the conventional TFT will be described below with reference to the application of this TFT to an active matrix type liquid crystal display device.
The TFT functions as a switching element for selectively addressing electric charges in the pixels arranged in a matrix pattern. It is, therefore, required to be capable of amply addressing electric charges in the pixels while in the ON state and retaining electric charges already addressed in the pixels far a required duration while in the OFF state. As a natural consequence, it is important that the TFT should be furnished with an ample ON/OFF ratio for the sake of fulfilling the switching function. The TFT used in a transmissive type liquid crystal display device is exposed to incident light on account of the operating principle. The TFT used in the reflective type liquid crystal display device is similarly exposed to incident light. The TFT which uses amorphous silicon or polycrystalline silicon, therefore, gives birth to carriers excited by light and tends to generate a leakage current particularly in the OFF state. The idea of lowering the magnitude of the leakage current generated during the OFF state and consequently securing an ample ON/OFF ratio has now become an essential technique for the TFT. If no large ON/OFF ratio is secured, the pixels in the case of a normally white mode, for example, become white and are perceived as a defect of the display device. When the TFT is used in a state inevitably exposed to incident light by reason of construction or environment of use as when it is used for a liquid crystal display device, the defective display due to such a decline of the ON/OFF ratio of the TFT as mentioned above tends to occur.
As a measure to lower the magnitude of the leakage current and secure an ample ON/OFF ratio, the provision of a black matrix or a shielding layer aimed at keeping the TFT from exposure to an incident light is now under contemplation. Also, the feasibility of a method for increasing the storage capacity of an pixel electrode to the extent of rendering the leakage current of the TFT negligible has been contemplated. U.S. Pat. No. 5,051,800 discloses a method which comprises using a shielding layer formed of a source electrode layer and a drain electrode layer for the sake of preventing impingement of light on the semiconductor layer.
When a black matrix of the conventional construction is disposed on the counter electrode side, however, since the black matrix and the TFT are separated by an intervening gap of several .mu.m and a liquid crystal composition is interposed between their opposed surfaces, the backlight and the light emanating from the environment around the display device are caused to impinge on the TFT owing to their diffused reflective inside the liquid crystal device. Though the method which resorts to enlargement of the area of a black matrix has been known to the art, it decreases the aperture ratio of the liquid crystal display device and induces deterioration of the quality of image. In the case of the method which resides in disposing a light shielding layer directly on the TFT, the shielding layer affects the operation of the TFT with its potential and, therefore, renders the decision of this potential difficult and entails the possibility of forming a short circuit between the shielding layer and the TFT. It also increases the number of steps of the process of production and complicates the construction of the device. It, therefore, has a problem of degradation of the yield of production of devices using the TFT.
The method which resorts to enlargement of the storage capacity of an pixel electrode entails a problem of degradation of the aperture ratio of a liquid crystal display device and a problem of necessity of imparting to the TFT high mobility for enabling electric charges to be addressed in pixels proportionately to the enlargement of the storage capacity.
The method which uses a shielding layer formed of a source electrode layer and a drain electrode layer for the sake of preventing impingement of light on the semiconductor layer has a problem of inability to lower the leakage current sufficiently because the semiconductor layer touches the source electrode layer and the drain electrode layer through the medium of a contact layer at the leading end of the shielding layer in the channel area.
The conventional techniques, as described above, are at a disadvantage in not easily lowering the leakage current of the TFT with efficiency.